Thanks to @Senk-Ju we finally got a well-written documentation of the LiquidBounce Nextgen ScriptAPI.
Check it out: https://liquidbounce.net/docs/Script API/Getting Started

You might also want to use https://maven.fabricmc.net/docs/yarn-1.21.1+build.3/index.html to get a better understanding on how to work with Minecraft.